The aim of the study is to evaluate the efficacy and safety of SKB264 as first-line treatment for patients with unresectable recurrent or metastatic triple-negative breast cancer (TNBC) whose tumors do not express programmed cell death ligand 1 (PD-L1) or in patients with PD-L1 positive tumors who received prior anti-programmed cell death 1 (PD-1)/PD-L1 inhibitor in early setting

This is a randomized, open-label, multicenter, Phase 3 study to evaluate the efficacy and safety of SKB264 versus investigator's choice chemotherapy as first-line treatment for patients with unresectable recurrent or metastatic TNBC whose tumors do not express PD-L1 or in patients with PD-L1 positive tumors who received prior anti-PD-1/PD-L1 inhibitor in early setting.

Key Inclusion Criteria:

1. Histologically and/or cytologically confirmed TNBC.
2. De novo metastatic or relapsed ≥ 6 months post completion of treatment with curative intent.
3. No prior systemic anti-cancer therapy for unresectable recurrent or metastatic disease.
4. Participants whose tumours are PD-L1-negative, or participants whose tumors are PD-L1 positive and have relapsed after prior anti-PD-1/PD-L1 inhibitor for early-stage disease.
5. At least one measurable lesion per RECIST v1.1.
6. Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1 with no worsening within 2 weeks prior to randomization.
7. A life expectancy of at least 3 months.
8. Eligible for the chemotherapy options listed as investigator's choice chemotherapy (paclitaxel, nab-paclitaxel, capecitabine, eribulin, or carboplatin) as assessed by the investigator.
9. Adequate organ and bone marrow function.

Key Exclusion Criteria:

1. Active second malignancy.
2. Uncontrolled or clinical significant cardiovascular disease.
3. History of noninfectious pneumonitis/interstitial lung disease (ILD) that required steroids or has current pneumonitis/ILD.
4. Active infection requiring systemic therapy within 2 weeks of randomization.
5. Active hepatitis B or hepatitis C virus infection.
6. Human immunodeficiency virus (HIV) positive or history of acquired immunodeficiency syndrome (AIDS); known active syphilis infection.
7. Known hypersensitivity to SKB264 or its excipients.
8. Previously received TROP2-targeted therapy or topoisomerase 1 inhibitors.
9. Prior treatment with the same investigator's choice chemotherapy (except taxane).
10. Pregnant or lactating women.